The present disclosure relates to a technique for generating a driving current for a light source, which emits a light beam for forming an electrostatic latent image, on the basis of an analog signal generated by smoothing a periodic pulse signal indicating a light amount of the light beam.
Formation of an image by an electrophotographic system includes a step of forming an electrostatic latent image of an image indicated by image data on a photosensitive drum, a step of supplying a toner to the electrostatic latent image to form a toner image, a step of transferring the toner image onto a sheet, and a step of fixing the toner image, which is transferred onto the sheet, on the sheet.
In the step of forming an electrostatic latent image, an electrostatic latent image is formed on the photosensitive drum by repeatedly reflecting a light beam, which is emitted from a light source by subjecting the light source to light emission control, with a polygon mirror and rendering a main scanning line on the rotating photosensitive drum.
When the magnitude of a driving current for the light source is fixed and the main scanning line is rendered on the photosensitive drum, a light amount (in other words, intensity) of the light beam irradiated on the photosensitive drum is different according to a position on the photosensitive drum. As a cause of the difference in the light amount of the light beam, for example, the distance between the photosensitive drum and the polygon mirror is different in the center and at both the ends of the photosensitive drum (the distance between the polygon mirror and the center of the photosensitive drum is shorter than the distance between the polygon mirror and both the ends of the photosensitive drum). An optical characteristic of a condensing lens arranged between the polygon mirror and the photosensitive drum is also a cause of the difference in the light amount of the light beam.
When the light amount of the light beam irradiated on the photosensitive drum is different according to the position on the photosensitive drum, unevenness occurs in the density of an image.
Therefore, the magnitude of the driving current for the light source is adjusted during main scanning to fix, on the photosensitive drum, the light amount of the light beam irradiated on the photosensitive drum. For example, there is proposed a technique for dividing the main scanning line into a plurality of blocks (areas), generating a pulse width modulation (PWM) signal indicating, for each of the areas, a light amount of a light beam emitted by the light source, smoothing the PWM signal, generating an analog signal having magnitude corresponding to the light amount of the light beam, and adjusting the magnitude of the driving current for the light source on the basis of the analog signal.
A duty ratio (i.e., a light amount of the light beam) of the PWM signal is switched among the areas. Therefore, image quality is sometimes affected (e.g., density unevenness of an image). In order to reduce this influence, there is proposed a technique for changing dividing positions of the areas for each main scanning line and diffusing the density unevenness of the image and a technique for finely setting output width of the PWM signal and controlling a light intensity setting value.
Control for switching the light amount of the light beam is performed in the vicinities of block boundaries. When the light amount of the light beam is suddenly changed, the image quality of regions equivalent to the vicinities of the block boundaries in the image (hereinafter, the image quality of the vicinities of the block boundaries) is sometimes deteriorated.
The sudden change of the light amount of the light beam in the vicinities of the block boundaries is one of events that are likely to affects the image quality of the vicinities of the block boundaries. As such an event, besides, there are, for example, ripples of an analog signal and a development characteristic γ. The ripples of the analog signal mean ripples that occur in the analog signal according to a period of the PWM signal when the PWM signal is smoothed to generate the analog signal. The development characteristic γ means that a change in the light amount of the light beam and a change in the density of the image are not in a proportional relation but in a logarithmic relation.
According to a state of an image to be formed, it is sometimes desired to give priority to suppression of the sudden change in the light amount of the light beam in the vicinities of the block boundaries, give priority to suppression of the influence of the ripples of the analog signal, or give priority to suppression of the influence of the development characteristic γ. It is convenient if an event to be coped with can be selected according to the state of the image to be formed out of a plurality of events that are likely to affect the image quality of the vicinities of the block boundaries.
It is an object of the present disclosure to provide an image forming apparatus that can select an event to be coped with out of a plurality of events that are likely to affect the image quality of the vicinities of the block boundaries.